In general, conductivity sensors that perform measurements according to an inductive or a conductive principle are used to measure the electrical conductivity of a medium in process automation.
For example, conductive conductivity sensors with at least two electrodes immersed in the medium to take measurements are known from the prior art. In order to determine the electrical conductivity of the medium, one determines the resistance or conductance of the electrode measuring path in the medium. If the cell constant is known, this information then serves to detect the conductivity of the medium. It is absolutely necessary that, in order to be able to measure the conductivity of a medium with a conductive conductivity sensor, at least two electrodes must come into contact with the measuring liquid.
The inductive principle of measuring the conductivity of process media assumes the use of sensors that are equipped with both a transmitter coil and a receiver coil installed at a distance from the transmitter coil. The transmitter coil produces an alternating electromagnetic field, which affects charged particles, e.g., ions, in the liquid medium and creates a corresponding electric current in the medium. As a result of this electric current, an electromagnetic field appears at the receiver coil, inducing a received signal (induction voltage) on the receiver coil according to Faraday's law of induction. This received signal can be analyzed and used to determine the electrical conductivity of the liquid medium.
Inductive conductivity sensors are typically constructed as follows: The transmitter coil and the receiver coil are, as a rule, built as toroidal coils, enveloping a continuous opening through which the medium can be applied, so that both the coils are encompassed in the medium. The excitation of the transmitter coil creates in the medium a closed current path that follows through both the transmitter coil and the receiver coil. By analyzing the current and voltage signals obtained at the receiver coil in response to the signal from the transmitter coil, the conductivity of the medium liquid can be determined. This principle is well established in industrial process measurement equipment, and has been documented in a large number of texts and patent literature—for example, in German Patent, DE 198 51 146 A1.
The Endress+Hauser group distributes such inductive conductivity sensors under such names as CLS50D, CLS54D, and CLD18.
In order to determine conductivity, it is necessary to know the temperature of the medium to be measured; as a rule, the conductivity increases with an increase in temperature. To measure temperature, a temperature sensor immersed in the medium, e.g., a Pt100, Pt1000, PTC, etc., is used. The temperature sensor is mostly situated in the casing, and comes into contact with the medium via an opening in the casing. Despite appropriate sealing measures, this opening creates a risk of leakage or leaks. This also means that more effort is required in implementing hygiene.
Even if the temperature sensor is not installed in the casing, it—like the part of a conductivity sensor that contacts the medium—must be connected via a flange leading to the vessel, in general, a process connection with a higher-level unit outside the medium. Here, as well, there is risk of leakage.